[Rn] 5f¹² 7s²

Here's how to understand this:

* [Rn] represents the electron configuration of Radon (Rn), the noble gas that comes before Fermium on the periodic table.

* 5f¹² indicates that there are 12 electrons in the 5f subshell.

* 7s² indicates that there are 2 electrons in the 7s subshell.

This means that Fermium has a total of 100 electrons (86 from Radon, 12 from 5f, and 2 from 7s).